1. Field of the Invention
The present invention relates to an engine bracket that is used for mounting of an engine mount installed between a vehicle body and a power unit in order to join the vehicle body and power unit to each other.
2. Description of the Related Art
Generally, an engine mount, which is installed on a vehicle to elastically mount the power unit on the vehicle body, is attached to the power unit or the vehicle body via an engine bracket. That is, an engine bracket is used to attach an engine mount to the power unit or the vehicle body in order to deal with the various configuration of an engine mount attachment portion of such a power unit and a vehicle body.
A power unit supporting structure allowing the power unit to be detached from the vehicle body during collisions has been scrutinized recently as a means for improving passenger safety during automotive vehicle collisions. As one measure for realizing such a power unit supporting structure, there has been proposed an engine bracket that is broken upon significant impact load during collisions, thereby allowing the power unit to be detached. A specific example has been disclosed in JP-A-2003-327155.
The engine bracket disclosed in JP-A-2003-327155 is such that notches are formed in the inner peripheral surface of bolt tightening holes, which are secured to the power unit, so that the bolt tightening holes are broken relatively easily by the impact load during vehicle collisions, breaking the connection by the engine bracket between the power unit and vehicle body, and thereby allowing the power unit to split off from the vehicle. However, investigation by the present inventors revealed that the engine bracket with the conventional structure described in JP-A-2003-327155 does not allow the required performance to be achieved very well.
That is, before becoming broken by the input of collision load during vehicle collisions as described above, the engine brackets must function to allow the power unit to be joined and supported in a stable manner against the vehicle body (engine mount) during ordinary driving conditions. In the engine bracket disclosed in JP-A-2003-327155, however, notches are formed in the inner peripheral surface of the bolt tightening holes, which are the seating surface of the fixing bolts, and these notches inevitably reduce the area of the surface on which the tightening force of the fixing bolts may act. Particularly, when larger sized, or greater numbers of, notches must be provided in the inner peripheral surface of the bolt tightening holes in order to more consistently ensure that the intended breakage will occur, a resulting concern is that it may become more difficult to sufficiently increase the bolt tightening force, and more difficult to ensure a high level of reliability by ensuring stable power unit support strength over a long period of time.
To effectively ensure power unit support strength and corresponding power unit support reliability, the size and number of the notches formed in the inner peripheral surface of the bolt tightening hole of the engine bracket may sometimes become limited, with the resulting risk that it may become more difficult to ensure the target breakage during vehicle collisions and more difficult to ensure that the power unit will consistently be detached. This in turn results in the risk of greater difficulties in effectively improving passenger safety during collisions.
Furthermore, because notches are formed in the bolt seat surface in the engine bracket described in JP-A-2003-327155, the effect in decreasing the break strength by the notches is substantially affected by the bolt tightening strength relative to both sides of the bolt tightening hole. That is, despite the formation of the notches, when the fixing bolts are tightened in an extremely firm manner, a resulting risk is that the intended breakage might not come about because of the high strength manifested by the bolt as a rigid body in the seat surface as a whole. On the other hand, if the fixing bolt is tightened in a relatively loose manner, breakage tends to occur as a result of the increase in the substantial stress concentration factor in the portions where the notches are located, without any show of strength by the bolt as rigid body in the seat surface as a whole. Consequently, differences in the strength by which the fixing bolt is tightened to the bolt seat surface can result in different breakage results, making it difficult to ensure consistent target breaking performance.